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</o:shapelayout></xml><![endif]--></head><body lang=KO link="#0563C1" vlink="#954F72"><div class=WordSection1><p class=MsoNormal><span lang=EN-US>Hi,<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US>Today I found <a href="https://stackoverflow.com/questions/5580140/is-there-a-way-to-tell-gcc-not-to-optimise-a-particular-piece-of-code">https://stackoverflow.com/questions/5580140/is-there-a-way-to-tell-gcc-not-to-optimise-a-particular-piece-of-code</a>. <o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US>So for functions you want to debug with -O0 option, place <o:p></o:p></span></p><p style='margin:0cm;margin-bottom:.0001pt;text-indent:22.0pt'><span lang=EN-US style='font-size:11.0pt;font-family:"Malgun Gothic";color:black'>#pragma GCC push_options<o:p></o:p></span></p><p style='margin:0cm;margin-bottom:.0001pt;text-indent:22.0pt'><span lang=EN-US style='font-size:11.0pt;font-family:"Malgun Gothic";color:black'>#pragma GCC optimize ("O0")<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US>In front of the function, and place<o:p></o:p></span></p><p class=MsoNormal style='text-indent:22.0pt'><span lang=DE style='font-size:11.0pt;color:black'>#pragma GCC pop_options</span><span lang=DE><o:p></o:p></span></p><p class=MsoNormal><span lang=DE>This sure works.<o:p></o:p></span></p><p class=MsoNormal><span lang=DE>Chan<o:p></o:p></span></p><p class=MsoNormal><span lang=DE><o:p> </o:p></span></p><div style='border:none;border-left:solid blue 1.5pt;padding:0cm 0cm 0cm 4.0pt'><div><div style='border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0cm 0cm 0cm'><p class=MsoNormal align=left style='text-align:left;text-autospace:ideograph-numeric ideograph-other;word-break:keep-all'><b><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri",sans-serif'>From:</span></b><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri",sans-serif'> ckim@etri.re.kr <ckim@etri.re.kr> <br><b>Sent:</b> Wednesday, October 6, 2021 11:58 AM<br><b>To:</b> kernelnewbies@kernelnewbies.org<br><b>Subject:</b> error: impossible constraint in 'asm' when compiling kernel code with -O0 option<o:p></o:p></span></p></div></div><p class=MsoNormal align=left style='text-align:left'><span lang=EN-US><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US>In linux-5.4.21 code,<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US>To prevent some variables from being </span>‘<span lang=EN-US>optimized out</span>’ <span lang=EN-US>during kernel code analysis using gdb, I added in drivers/iommu/Makefile,<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US>(replacing -O2 flags to -O0 flags for some files, I found <a href="http://www.joelfernandes.org/linux/2018/06/10/kernel-gdb.html">http://www.joelfernandes.org/linux/2018/06/10/kernel-gdb.html</a> )<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US>CFLAGS_REMOVE_arm-smmu.o := -O2<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US>CFLAGS_REMOVE_arm-smmu-impl.o := -O2<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US>CFLAGS_REMOVE_io-pgtable-arm-v7s.o := -O2<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US>CFLAGS_REMOVE_io-pgtable-arm.o := -O2<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US>CFLAGS_REMOVE_of_iommu.o := -O2<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US>CFLAGS_REMOVE_arm-smmu-v3.o := -O2<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US>CFLAGS_arm-smmu.o := -O0<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US>CFLAGS_arm-smmu-impl.o := -O0<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US>CFLAGS_io-pgtable-arm-v7s.o := -O0<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US>CFLAGS_io-pgtable-arm.o := -O0<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US>CFLAGS_of_iommu.o := -O0<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US>CFLAGS_arm-smmu-v3.o := -O0<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US>But when I do make ARCH=arm64 CROSS_COMPILE=aarch64-none-elf- Image, I get<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd;text-indent:10.0pt'><span lang=EN-US>CALL scripts/checksyscalls.sh<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US> CALL scripts/atomic/check-atomics.sh<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US> CHK include/generated/compile.h<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US> CC drivers/iommu/io-pgtable-arm.o<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US>In file included from ././include/linux/compiler_types.h:68,<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US> from <command-line>:<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US>./arch/arm64/include/asm/jump_label.h: In function 'system_uses_lse_atomics':<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US>./include/linux/compiler-gcc.h:120:38: warning: asm operand 0 probably doesn't match constraints<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US> 120 | #define asm_volatile_goto(x...) do { asm goto(x); asm (""); } while (0)<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US> | ^~~<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US>./arch/arm64/include/asm/jump_label.h:38:2: note: in expansion of macro 'asm_volatile_goto'<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US> 38 | asm_volatile_goto(<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US> | ^~~~~~~~~~~~~~~~~<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US>./include/linux/compiler-gcc.h:120:38: warning: asm operand 0 probably doesn't match constraints<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US> 120 | #define asm_volatile_goto(x...) do { asm goto(x); asm (""); } while (0)<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US> | ^~~<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US>./arch/arm64/include/asm/jump_label.h:38:2: note: in expansion of macro 'asm_volatile_goto'<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US> 38 | asm_volatile_goto(<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US> | ^~~~~~~~~~~~~~~~~<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US>./include/linux/compiler-gcc.h:120:38: error: impossible constraint in 'asm'<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US> 120 | #define asm_volatile_goto(x...) do { asm goto(x); asm (""); } while (0)<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US> | ^~~<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US>./arch/arm64/include/asm/jump_label.h:38:2: note: in expansion of macro 'asm_volatile_goto'<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US> 38 | asm_volatile_goto(<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US> | ^~~~~~~~~~~~~~~~~<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US>./include/linux/compiler-gcc.h:120:38: error: impossible constraint in 'asm'<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US> 120 | #define asm_volatile_goto(x...) do { asm goto(x); asm (""); } while (0)<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US> | ^~~<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US>./arch/arm64/include/asm/jump_label.h:38:2: note: in expansion of macro 'asm_volatile_goto'<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US> 38 | asm_volatile_goto(<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US> | ^~~~~~~~~~~~~~~~~<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US>make[2]: *** [scripts/Makefile.build:266: drivers/iommu/io-pgtable-arm.o] Error 1<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US>make[1]: *** [scripts/Makefile.build:509: drivers/iommu] Error 2<o:p></o:p></span></p><p class=MsoNormal style='margin-left:20.0pt;mso-para-margin-left:2.0gd'><span lang=EN-US>make: *** [Makefile:1652: drivers] Error 2<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US>Does anybody know how can I use -O0 flag? (I tried -Og flag but some variables are still optimized out).<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US>Thank you!<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US>Chan Kim<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US><o:p> </o:p></span></p></div></div></body></html>